Heretofore, a film of various metals such as Au, Ag and Pt and Cu; a film of metal oxides such as indium oxide doped with tin or zinc (ITO or IZO), zinc oxide doped with aluminum or gallium (AZO or GZO) and tin oxide doped with fluorine or antimony (FTO or ATO); a conductive film of nitrides such as TiN, ZrN and HfN and a conductive film of borides such as LaB6 and the like are well-known as transparent conductive electrodes, and further, various electrodes including a combination thereof such as Bi2O3/Au/Bi2O3 and TiO2/Ag/TiO2 and the like are well-known. In addition to the transparent electrodes described above, a transparent electrode employing CNT (carbon nanotube) or a conductive polymer has been also proposed (for example, refer to “Technologies of Transparent Conductive Film”, p. 80, published by Ohmsha, Ltd.).
However, the films of metals, nitrides or borides described above and a conductive polymer film are utilized only in a specific technological field such as electromagnetic wave shielding or in a touch panel field where even a relatively high resistance is acceptable, since high optical transparency and high conductivity are incompatible.
On the other hand, a metal oxide film is predominantly utilized as a transparent electrode, since it has compatibility between high optical transparency and high conductivity and excellent durability. Particularly, ITO is often utilized as a transparent electrode for various optoelectronics application, since it has good balance between optical transparency and conductivity, and can easily form a fine electrode pattern according to a wet process employing a solution as well as a vacuum process such as sputtering. However, the vacuum process such as sputtering requires expensive equipment in order to form a transparent conductive film, while the wet process requires annealing treatment at high temperature of 500° C. or higher in order to obtain high conductivity.
Besides the transparent electrodes described above, there are proposed a transparent conductive substrate a having random network structure comprised of self-organizing silver particles (for example, refer to WO 2007/114076) and a transparent electrode comprised of metal nanowires with fine meshes (for example, refer to US Patent Publication No. 2007/0074316). Particularly, metal nanowires employing silver nanowires provide compatibility between high conductivity and high transparency, due to high conductivity which silver itself.
In order to use a transparent electrode as an electrode for various devices such as an LCD, an electroluminescence element, a plasma display, an electrochromic display, a solar cell and a touch panel, it is necessary to form a pattern of the transparent electrode. As a method of forming a transparent electrode pattern employing metal nanowires, there are mentioned a method employing a printing ink containing electroconductive microwires (for example, refer to Japanese Patent O.P.I. Publication No. 2003-515622) and a method forming a nanowire pattern according to photolithography (for example, refer to US Patent Publication Nos. 2005/0196707 and 2008/0143906).
However, a method of directly pattern printing metal nanowires lowers electroconductivity due to increase of contact resistance between the metal nanowires caused by a binder. Patterning according to a conventional etching method comprises the steps of forming a resist pattern by a photoresist, imagewise exposing, developing, removing the resist by etching, and post-processing. In such a patterning, many steps before and after etching are required, and etching of the resist is carried out in a solution and therefore is likely to cause expansion or separation of the resist, which may result in lowering of etching accuracy. Further, severe temperature control is required. An etching solution used for etching a silver film is composed mainly of ammonia, which releases a strong odor and pollutes the working environment. A strong acid etching solution such as nitric acid has problems in that it has an adverse influence on the resist and releases a highly toxic gas. Furthermore, removal of the resist penetrating between the fine metal nanowire meshes may be insufficient, resulting in lowering of light transmittance, and on removal of the resist, metal particles or metal nanowires are also released. As described above, a conventional pattern formation method is not satisfactory. Dry etching, which is an etching method other than photolithography, enables high precision patterning, but is low in processing speed and in processing capability of the processing apparatus, resulting in cost increase. Besides the above, an etching method is disclosed in for example, Japanese Patent No. 3173318, in which a paste containing particulate substances and a solution having etching capability is applied to a material to be etched, however, this method requires high temperature and is not applied to a film substrate. Further, there is no disclosure of metal particles or metal nanowires in that patent.